1. Field of the Invention
The present invention relates to a boundary microphone that is mainly placed on a desk upon use, and more specifically to a boundary microphone including a metal mesh to improve a shielding effect against electromagnetic waves.
2. Description of the Related Art
Boundary microphones are known that are mainly placed on a desk upon use. FIG. 5 illustrates an example of a conventional boundary microphone. A base 17 of the boundary microphone illustrated in FIG. 5 is formed of a flat piece of metal and can be fixed on a surface of a desk or a floor. In an upper surface of the base 17, a cavity (hollow) for mounting a circuit board 18 and the like is formed. From the upper surface of the base 17, a column 21 integrally formed with the base 17 is erected, approximately in the center when viewing the base 17 from the plane direction. A later described cover 20 is connected to the base 17 via the column 21. In the column 21, a screw hole 12 is formed from the upper end. For the base 17, the left side and the right side in FIG. 5 are the front side and the backside, respectively, and a wall is integrally formed at the back end of the base 17. A bush 19 is fitted into a hole that is formed through this wall. In the case of a boundary microphone installed in a conference room, the boundary microphone is installed on a desk or the like with the front side facing the participants. One end of a microphone cord 15 is passed through a center hole of the bush 14. The microphone cord 15 is generally a cord constituted of a two-core balanced output cord and a shielding wire wrapping around the two-core balanced output cord. The end portions of the above-described two-core signal cable and shielding wire constituting the microphone cord 15 are electrically connected to a predetermined soldering land or the like on the later-described circuit board 18.
In the upper surface of the base 17, the circuit board 18 is secured to the approximately front half portion of the above-described cavity so as to plug the cavity. In the upper surface of the circuit board 18, a microphone unit 16 is mounted with the sound wave introducing port thereof facing forward. As the microphone unit 16, a capacitor microphone unit is generally used. The cover 20 for covering the whole upper surface of the base 17 including the microphone unit 16 and the circuit board 18 is put on the base 17. The cover 20 is made of a metal material, as in the case of the base 17, and numerous openings for introducing sound waves to the microphone unit 16 are formed therein. As the material of the cover 20, generally, a punching metal is used, in which numerous holes are formed by punching. The cover 20 is press molded into a flat plate form which is then inverted and put on the upper surface of the base 17. In the cover 20, approximately in the center when viewing the cover 20 in the plane direction, a hollow is formed at a position corresponding to the column 11 of the base 17 and a hole is formed in the bottom of this hollow. A screw 13 as a fastening member is inserted into this hole, and the screw 13 is screwed into the screw hole 12 formed in the column 11, so that the cover 20 is fastened to the base 17. The head of the screw 13 sinks into the inside of the hollow of the cover 20. A receiving portion for the peripheral portion of the cover 20 is formed in the periphery of the upper surface side of the base 17, and, the receiving portion is designed so that the peripheral portion of the cover 20 can be in contact with the base 17 while the cover 20 is fastened to the base 17 with the screw 13 as described above.
As described above, the boundary microphone includes mainly two parts, i.e., the base 17 and the cover 20 in appearance, and the internal components are incorporated in the internal space. The screw 13 inserted in the hole of the cover 20 is screwed into the screw hole 22 of the base 17, and whereby the base 17 and the cover 20 are mutually fastened. The base 17 and the cover 20 are fastened together with one screw 13 approximately in the center of the cover 20 and the head of the screw 13 fits into the hollow of the cover 20.
In the internal space enclosed by the base 17 and the cover 20, electric circuits, such as an impedance converter, a tone control circuit, and an output circuit, as well as the microphone unit 16 and the circuit board 18 are incorporated. If high frequency noise composed of electromagnetic waves enters these electric circuits from the outside, this noise is detected by a semiconductor element used in the impedance converter or the like, and is then mixed into a sound signal as a noise signal, thereby degrading the signal to noise ratio (S/N) at the microphone output. Accordingly, it is preferable that the connecting portions in the peripheral portions of the base 17 and the cover 20 are joined together without any gap so as to shield the internal components from high frequency noise coming from the outside. The reason is that if these connecting portions are not joined together properly and the base 17 and the cover 20 are in point contact with each other and there is a gap therebetween, high frequency noise will enter through this gap.
Unfortunately, the conventional boundary microphone has such a structure that the base 17 and the cover 20 tend to be in point contact with each other for the reason described below. The base 17 is typically manufactured using a zinc die casting method or the like and has an uneven casting surface. On the other hand, for the cover 20, a punching metal is typically used as the material as described above, and by press molding this, the cover 20 is formed into a desired shape. However, the peripheral portion, which is electrically and mechanically in contact with the base 17, is not flat and there occur irregularities. Accordingly, if the cover 20 is directly put on the base 17 and is screwed thereinto with the screw 13, the base 20 and the cover 17 will be in point contact with each other.
The electromagnetic waves of VHF and UHF bands, which are used in the conventional TV broadcast and the like, can be shielded even if the base 17 and the cover 20 are in point contact with each other. In recent years, mobile phones using radio waves in a shorter wavelength region are widely used. There is a problem that the electromagnetic waves having a short wavelength can enter the internal space of a microphone even through a very small gap. In addition, the mobile phone is used by user's side and therefore the mobile phone is more frequently used near a microphone. Thus there is more chance of electromagnetic wave noise entering the microphone. This is true of the boundary microphone that is used in a conference room and the like.
In view of such a circumstance, Japanese Patent Application Publication No. 2009-100157 discloses an invention allowing a shielding effect to be exhibited even if a mobile phone is used near a boundary microphone. A boundary microphone according to the invention disclosed in Japanese Patent Application Publication No. 2009-100157 includes: a first metallic part disposed on one side of the upper and lower sides of internal components; and a second metallic part which is disposed on the other side of the upper and lower sides of the internal components and which encloses the internal components along with the first metallic part in all directions. A base, a cover, the first metallic part, and the second metallic part are alternately overlapped at their peripheral portions, and at least one of the first metallic part and the second metallic part is made of a metal mesh in order to pass sound waves to a microphone unit.
According to the invention disclosed in Japanese Patent Application Publication No. 2009-100157, the internal components are disposed in the space enclosed by the base and the cover and furthermore the internal components are enclosed by the base, the cover, the first metallic part, and the second metallic part whose peripheral portions are alternately overlapped. Thus, the electromagnetic waves which are to enter the internal components from the outside can be blocked more effectively. For example, even if a mobile phone is used near the microphone, it is possible to prevent the electromagnetic wave from entering the internal components and also possible to prevent the occurrence of noise caused by the electromagnetic wave. The shielding effect can be further enhanced by overlapping the peripheral portions of the first metallic part and the second metallic part with each other on the inside and outside thereof and by disposing the internal components in a space enclosed by the first metallic part and the second metallic part.
Still, there is a room for improvement for the shielding effect of the boundary microphone of the invention disclosed in Japanese Patent Application Publication No. 2009-100157 because the first metal mesh and the second metal mesh overlap with each other and a gap may be produced therebetween. The metal meshes and a boundary plate can be partially electrically connected with a screw and the like, but their connection is not continuous. Thus, there is a room for improvement for surely preventing electromagnetic waves from entering.